When an electronic system includes multiple cache memories, the validity, or coherence, of the data held in the system is maintained to ensure integrity of data operations. Coherency is often accomplished by manipulating data according to a cache coherency protocol. As the number of caches and/or processors increases, the complexity of maintaining coherency increases, which potentially results in a greater number of conflicts between the multiple components of a system.
For example, when multiple components (e.g., a cache memory, a processor) request the same block of data, a conflict between the multiple components is resolved in a manner that maintains validity of the data. Previous cache coherency protocols typically have a single component that is responsible for conflict resolution. However, as the complexity of the system increases, reliance on a single component for conflict resolution can decrease overall system performance.
Generally, there are two basic schemes for providing cache coherence: snooping (now often called Symmetric MultiProcessing SMP) and directories (often called Distributed Shared Memory DSM). The fundamental difference between these two schemes revolves around the placement and access to meta-information; that is, the information about where copies of a cache line are held.
For directory-based schemes, in addition to a fixed place where the uncached data is stored, there is a fixed location, the directory, indicating where cached copies reside. In order to access a cache line in a new way, a node must communicate with the node containing the directory, which is usually the same node containing the uncached data repository, thus allowing the responding node to provide the data when the main storage copy is valid. Such a node, in a directory system, is referred to as a home node.
The directory may be distributed in two ways. First, main storage data (the uncached repository) is often distributed among nodes, with the directory distributed in the same way. Secondly, the meta-information itself may be distributed, keeping at the Home node as little information as whether the line is cached, and if so, where a single copy resides. SCI, for example, uses this scheme, with each node that contains a cached copy maintaining links to other nodes with cached copies, thus collectively maintaining a complete directory.
For snooping caches, the meta-information is distributed with the cached copies themselves, such that each valid copy of a cache line is held by a unit, which is to recognize requests to access the cache line and respond accordingly. Typically, there is a repository where the data is held in an un-cached state, i.e. its original location. Often, an agent or unit responsible for this original location is referred to as a home node. When requesting access to a cache line, a requesting node often broadcasts the address of a requested cache line, along with permissions needed, and nodes holding a copy of the requested cache line respond accordingly.
However, in some of the current coherence and messaging protocols partial reads of cache lines and non-snoop memory accesses, i.e. noncoherent accesses in a coherent architecture, under certain circumstances potentially result in forwarding of invalid data to a requestor. Often these circumstances arise from violations of memory ordering rules within a protocol. As a result, illegal operation may be performed by agents receiving the invalid data, which potentially leads to instability within a system.